The present invention pertains to the field of wrought metal base alloy products with improved chemical and physical characteristics, and more particularly to products of tantalum or niobium metal base alloys containing quantities of silicon and a dopant such as yttrium nitride.
Tantalum alloys have been recognized as preferred materials in the field of furnace equipment: such as trays and heating elements, and radiation shielding where the thermal stability of the alloy is maintained and the life span of the product is enhanced by reduced embrittlement. Tantalum alloys have also been employed in the manufacture of wire and more particularly as electric component leads where product characteristics such as ductility, high dielectric constant, resistance to grain growth at elevated temperatures, and improved processability are required. In the production of capacitors, for example, the lead wires may either be pressed into the tantalum powder anode and subsequently sintered at high temperatures, or spot welded to sintered capacitor bodies. See U.S. Pat. No. 3,986,869.
In both electrical component and furnace equipment products, contamination by oxygen contributes to embrittlement and piece failure. For example, in wire products, the area where a lead wire leaves an anode body is highly susceptible to embrittlement due to migration of oxygen from the sintered body to the wire. Lead wires which become embrittled or break results in the loss of the entire piece. Substantial economic benefit can be gained from a tantalum or niobium base alloy which does not lose strength or ductility due to embrittlement after exposure to high temperatures.
For purposes of simplicity, reference hereafter will be made solely to tantalum even though it is understood that niobium is also contemplated. The chemical similiarities between the two elements are well known to those skilled in the art.
The term "ductility" is typically understood to mean a percentage increase in length of the metal prior to failure in a tensile test.
The term "bend-ductility" is a physical characteristic synonymous with reduced embrittlement or ability to withstand repetitive bending. The term is typically represented as a number of successful bends in an anode after single or double sintering in vacuum.
Oxygen embrittlement occurs in tantalum base alloy products by several mechanisms. Tantalum acts as a getter for oxygen in addition to other gaseous impurities present in sintering operations such as carbon monoxide, carbon dioxide, and water vapor. Attempts have been made to reduce tantalum oxide formation by doping tantalum with carbon or a carbonaceous material. Oxygen reacts with the carbon at the surface of the metal rather than diffusing into the tantalum thereby minimizing embrittlement. While enhanced ductility levels may be achieved with carbon addition, the dopant may adversely effect the processability and electrical characteristics of the metal. Carbon particles on the surface of the tantalum may result in increased electrical leakage due to the non-uniform adherence of tantalum oxide film.
The term "dopant" is known to those skilled in the art to mean a trace quantity of material which is normally added to a base material.
The term "processability" is defined here after as the ratio of tensile strength to yield strength. Processability is measured by mechanical evaluation of tantalum alloy by a variety of methods including standardized ASTM testing referenced hereafter.
U.S. Pat. Nos. 4,128,421 and 4,235,629 disclose the addition of silicon and/or carbon to tantalum to increase ductility. Silicon is volatilized in part during processing and therefore must be added in excess in the original master blend.
While it is speculated that silicon functions as a getter similar to carbon, the addition of excess silicon may effect the electrical characteristics of the wire product by the same mechanism described above for carbon or carbonaceous materials.
The doping of tantalum powder with phosphorus is generally disclosed in U.S. Pat. Nos. 3,825,802, 4,009,007, and 4,957,541 as a means for improving the electrostatic capacity of capacitors and flow properties of the tantalum powders. Some significance is attributed to the amount of dopant added in the '007 patent (ranging from 5 to 400 ppm). Although the mechanism by which phosphorous functions as a dopant to tantalum metal is not completely known, one theory is that it reduces the sintering rate of tantalum by decreasing the surface diffusion of tantalum.
Another mechanism for reducing the embrittlement of tantalum base alloy products involves the doping of tantalum powder with yttrium, U.S. Pat. Nos. 3,268,328, 3,497,402; or thoria, U.S. Pat. No. 4,859,257; or oxides therefrom.
U.S. Pat. No. 3,268,328 discloses a yttrium oxide doped tantalum alloy having an average grain size of 4 to 6 (ASTM).
The term "grain-size" may be defined as the number of grains of tantalum as compared with a standard ASTM grain size chart at 100X magnification. The term "fine grain-size" may be defined to mean an ASTM value of greater than ASTM 5 or less than about 55 microns. The term "uniform grain-size" refers to a grain-size which does not vary by more than one ASTM number according to the testing procedure discussed above.
A combination of dopants in a tantalum base alloys for wrought wire applications is disclosed in U.S. Pat. No. 4,859,257. The patent discloses an alloy formed by adding 125 ppm silicon and 400 ppm thoria to tantalum powder. An ASTM grain size No. 10 and No. 5 are obtained for a doped and an undoped control of pure tantalum powder. This translates into a doped tantalum base alloy grain size of 10 microns in comparison to a control of 55 microns. It is maintained that the mechanisms where silicon functions as an oxygen getter and where metal oxide functions as a grain boundary restraint, explain the basis for the reported fine grain size and ductility. The mechanisms, however, suffer from previously discussed problems of product quality due to silicon evaporation and grain growth after exposure to high temperatures due to dispersant particle growth. A tantalum based alloy which provides consistently high ductility and processability after exposure to high temperatures would be a considerable advance in the field of tantalum metallurgy.
Another object of the present invention is to provide tantalum alloy which maintains processability and ductility with low concentrations of dopants.
A further object of the present invention is to provide a doped tantalum alloy which maintains a high level of processability and ductility and wherein the dopants resist coarsening after exposure to high temperatures.
Yet a further object of the present invention is to provide a wrought wire product from tantalum base alloy which maintains processability and ductility, and which minimizes DC electrical leakage.
Accordingly, the present invention alleviates the above mentioned problems and achieves the cited objectives in a wrought metal alloy product comprising a tantalum or niobium base metal, a quantity of silicon between about 10 to about 1000 ppm, and between about 10 to about 1000 ppm of a dopant comprising a metallic and a non-metallic component. The dopant has a Gibbs free energy of formation higher than compounds formed from the tantalum or niobium base metal selected and the non-metallic component of the dopant, and a Gibbs free energy of formation lower than oxides formed of the dopant metal component.
The present invention further comprises in a wrought metal alloy product, the combination of a tantalum or niobium base metal with about 100 to about 500 ppm silicon and about 100 to about 500 ppm yttrium nitride. The product further includes a ductility of about 20% after exposure to elevated temperatures of greater than 1300.degree. C., and exhibits a fine uniform grain size of about 3 to about 30 microns. Low levels of carbon and oxygen impurities are maintained at about 50 and 300 ppm respectively. As discussed below, the inventors have discovered that the unexpected physical and chemical properties of the invention are largely due to the synergistic effect of silicon and yttrium nitride dopants.
A further advantage is that yttrium silicide is more resistant to dispersant particle growth than metal oxides such as yttrium or thoriam oxides.
A further advantage of the present invention is that wrought metal alloy products produced have improved ductility after exposure to elevated temperatures and improved bend ductility.
A further advantage is that excess quantities of dopant formerly needed to replace evaporated silicon are not required. The grouping of excess dopant on the surface of the wrought alloy product and the associated problem of discontinuous tantalum oxide insulating, is also alleviated.